A typical air conditioning system for an automotive vehicle includes a compressor, an evaporator, a condenser (typically part of the vehicle's cooling module) and an expansion valve arranged in a direct expansion cooling circuit. Some of these systems include a phase change material built into the main evaporator which is used to provide cooling for a short period of time without running the compressor. Certain vehicles having start-stop functionality include such evaporators having built-in phase change material that surrounds refrigerant passages in the evaporators. It should be understood that this phase change material is not the refrigerant which is circulated through the air conditioning system by the compressor which is also a phase change material, typically changing phases between liquid and gas and back to liquid as it circulated around the direct expansion cooling circuit of the air conditioning system. In vehicles having start-stop functionality, an engine of the vehicle is off when the vehicle is at a stop. The phase change material in the main evaporator was cooled to change phase when the vehicle engine was running and the compressor thus running. For example, the phase change material is frozen from a liquid to a solid state. When the vehicle is at a stop and the engine not running, the phase change material absorbs heat from the evaporator and keeps the evaporator cool. Air flowing across the evaporator is thus continued to be cooled while the vehicle engine and thus the compressor of the air conditioning system is off. This provides fuel savings as there is no need to run the vehicle engine when the vehicle is at a stop to continue to maintain cool air flowing into the passenger cabin of the vehicle to cool it. Otherwise, if the vehicle is at a stop, and cooling is needed for the passenger cabin, the vehicle engine needs to be restarted to run the compressor of the air conditioning system to cool the passenger cabin. Such evaporators having built in phase change material typically provide a few minutes of cooling when the vehicle engine and thus the compressor is off. This is typically adequate to keep cool air flowing into the passenger cabin (if needed) for the periods of time that a vehicle is at a stop during normal driving, such as the time that a vehicle is stopped at a red light or a stop sign.
In vehicle air conditioning systems in which the phase change material is built into the main evaporator, the cooling effect is delayed when the air conditioning system is first turned. This is due to the need to cool the phase change material sufficient for it to change state to the lower enthalpy phase state, such as freezing to a solid. The same is the case after a stop-start event if the phase change material has absorbed sufficient heat so that all the phase change material has changed to the higher enthalpy phase state, such as melting to a liquid. Such systems also do not include a place to store energy during a deceleration or braking event. When a vehicle is decelerating, there is typically mechanical energy that in effect is in effect going to waste.